The attitude, that is to say the angular orientation, of a spacecraft such as a satellite or a probe on a predetermined trajectory is usually controlled by internal actuators such as inertia wheels making it possible to apply an internal torque to the spacecraft and to cause a rotation about one of its axes X, Y, Z, the axes X, Y, Z forming a reference trihedron associated with the spacecraft. The spacecraft tends to become misaligned under the action of the disruptive torques produced by the environment such as the solar pressure, aerodynamic friction forces, electromagnetic torques, and torques due to the gravity gradient. It is therefore necessary to actively control the angular orientation of the spacecraft and ensure stability of this orientation on its three axes. The attitude is permanently controlled by a feedback loop comprising sensors which measure the orientation of the spacecraft, an onboard computer which processes these measurements and establishes the commands that are executed by one or more actuators in order to counterbalance the drifts and maintain an orientation in a chosen direction. However, every time the wheels supply an internal torque, their speed increases up to a maximum speed called saturation speed. When the maximum speed is reached, the inertia wheels can no longer compensate for the drifts and the onboard computer then begins a wheel-unloading operation.
The wheels are usually unloaded by using additional external actuators applying to the spacecraft an external torque that is chosen so as to reduce the speed of the wheels until the wheels resume their original speed.
To unload the inertia wheels, it is known practice to use magneto torquer bars which, by interaction with the terrestrial magnetic field, create a magnetic torque in order to reduce the speed of the wheels. These external actuators work well for spacecraft placed in low earth orbit, LEO, about a planet with a magnetosphere such as the Earth, for altitudes usually of up to 2000 km for the Earth, because the intensity of the terrestrial magnetic field is high close to the planet but they work less well on average at higher altitudes. In addition, in equatorial earth orbit, the plane of the orbit is that of the terrestrial equator and the axis of the magnetic field is virtually orthogonal to the plane of the orbit. Since no magnetic torque can be created on the axis of the magnetic field, certain drifts of the satellite can therefore not be compensated for by these actuators.
At high altitude or in geostationary orbit GEO (Geosynchronous Earth Orbit), it is known practice to use thrusters to carry out the unloading of the inertia wheels. The thrusters make it possible to create an external torque by the emission of gas jets. However, the thrusters have the disadvantage of moving the spacecraft in rotation but also in translation which disrupts the orbit of the spacecraft, creates many vibrations and causes pointing losses. In addition, since the thrusters are usually placed on one of the sides of the spacecraft, it is necessary to have the spacecraft turn in rotation in order to correctly orient the thrusters during the operation for unloading the inertia wheels. Finally, the use of the thrusters to unload the wheels causes additional fuel consumption while the inertia wheels are powered electrically via the solar energy captured by solar panels with which the spacecraft is fitted.